Who Goes Home? by Sylvia Waugh

Like Patrick in Space Race and Matthew and Alison in Earthborn, Steven is a visitor from Ormingat, living on Earth with his family for a designated number of years. But Steven is not merely an observer, he is an arranger, a facilitator with the power to direct attention away from any Ormingatriga who needs protection. When his earthly son Jacob is born with a fatal immune deficiency, Steven requests help from Ormingat in order to save his life and as a result the boy survives, but grows up surrounded by a protective shield without friends. When Steven is forced to tell Jacob of his identity and involve him in his work, Jacob resents his father's imposition. Then comes the debacle of the Derwents' accident and Nesta's flight from home, which both bring undesired publicity and the danger of detection to the Ormingat aliens. Steven, summoned to return early to the mother planet, does not want to go. If he returns he must abandon his earth wife, Lydia. And where does all this leave unhappy Jacob who makes contact not only with Mrs Dalrymple but also Nesta's family in York? A strong conclusion to the trilogy, uniting the plots from the earlier books - with a surprising and dramatic finale.